Comprising of nine states, EU9 is now a significant part of the EU. The EU9 became part of the EU in its third expansion phase in 2004, bringing diversity and fresh perspectives into the EU society.

The members of EU9 include Hungary, Slovenia, Latvia, Lithuania, Poland, Estonia, Slovakia, Czech Republic, and Malta. These states have given meaningful contributions towards advancing the objectives of the European Union.

As such, comprehending the responsibility these nations plays in the EU is important for any study relating to international relations, politics, or European economics.

Right from their admission, the EU9 countries have contributed substantially towards shaping the European Union's policies. Their geographical location provides important strategic benefits, making them a vital part of the defense and security here strategies of the EU.

Additionally, these countries have significantly contributed to the European Union's economic strength. The EU9 nations' rapid economic growth and willing integration into the EU market has led to amplified trade with other EU countries.

The social contribution of the EU9 is also significant. These nations incorporate deep historical backgrounds, unique cultural practices, and diverse languages, therefore enhancing the plurality of the EU.

The introduction of EU9 has also influenced the internal politics within the EU. With their involvement, the power dynamics have altered, with new coalitions and alliances developing.

In conclusion, the roles played by the EU9 nations are substantial. From economic influence to cultural richness, and from security strategies to political dynamics, EU9 has robustly augmented the European Union's core strength. As such, an thorough understanding of the EU9 is crucial to fully grasping the subtleties of the European Union.